The present invention relates to an overload coupling device for thread-cutter chucks or quick-change inserts therefor.
An overload coupling device of the type under consideration comprises a cylindrical drive sleeve loadable with a rotational drive moment, a drive shank driven by the drive sleeve and held coaxially and rotatably with that sleeve, a mounting receiving a tap drill, and a torque coupling positioned in the force flux between the drive sleeve and the drive shank.
The overload coupling device of this type is disclosed in German patent DE-PS No. 25 11 148 corresponding to U.S. Pat. No. 4,041,729. In the disclosed coupling device the drive sleeve has three cage slots spaced from each other at equal intervals in the circumferential direction, a coupling ball being supported immediately in each of said cage slots with a sufficient motion play. The width of each cage slots corresponds, taking into consideration the motion play, to the diameter of the coupling ball. The wall of each cage slot, which upon the release of the ball-pocket coupling, lifts the coupling balls out from the respective ball pockets, is formed such that during the uncoupling the coupling balls remain in this uncoupled position and do not tend thereby, during the repeated passing the ball pockets, to fall back into these pockets under a short-time clutching-in of the coupling. The overload coupling device operates precisely and durably separates the quick-change insert from the drive upon the occurrence of the torque release. The construction of the plane-retaining axial surfaces of the spring-biased thrust ring acting on the coupling balls is particularly suitable; other specific advantages of the known overload coupling device are described in detail in the aforementioned patent.
It has been, however, shown that the specific shape of the respective walls of the cage slots, which lift or push the coupling balls away from their pockets, upon the release of the coupling, involved quite high manufacturing expenses and require high precision because these walls or surfaces are each divided into two individual surfaces, from which one individual surface is formed as the radially inwardly extending surface adjacent the peripheral surface of the drive shaft, particularly the contact portion, which in the coupled condition of the coupling abuts somewhat tangentially on the periphery of the coupling ball. The 15 other individual surface extends at an angle relative to that contact portion and thereby parallel to the opposite wall of the cage slot. Both surface portions together run in the corner region which lies substantially on the circumference which is greater than the outer circumference of the drive shaft and on which the coupling balls lie in the coupled condition. Such wall of the cage slot, which is subdivided into two individual surfaces inclined to each other, is difficult and expensive to manufacture. In the corner region the wall of the cage slot, where two surface portions strike against each other, terminates with a blade-shaped line. Upon the transition of the coupling balls from the coupled position into the uncoupled position, the coupling balls must pass this blade-shaped line, whereby forces will exert on the coupling balls in this region during the circumferential movement. This causes increased wear of the walls of the cage slots and of the coupling balls as well.